This invention generally relates to an ink jet printer that uses an oscillating microelectromechanical actuator to break up a fluid stream in a continuous inkjet printer, or to assist in the selective generation of microdroplets of ink in a drop-on-demand system.
Many different types of digitally controlled printing systems have been invented, and many types are currently in production. These printing systems use various actuation mechanisms, various marking materials, and various recording media. Examples of digital printing systems in current use include: laser electrophotographic printers; LED electrophotographic printers; DOT matrix impact printers; thermal paper printers; film recorders; thermal wax printers; dye diffusion thermal transfer printers; and ink jet printers. However, at present, such electronic printing systems have not significantly replaced mechanical presses, even though this conventional method requires very expensive set-up and is seldom commercially viable unless a few thousand copies of a particular page are to be printed. Thus, there is a need for improved digitally-controlled printing systems that are able to produce high-quality color images at a high speed and low cost using standard paper.
Ink jet printing is a prominent contender in the digitally controlled electronic printing arena because, e.g., of its non-impact, low-noise characteristics, its use of plain paper, and its avoidance of toner transfers and fixing. Inkjet printing mechanisms can be categorized as either continuous inkjet or drop-on-demand ink jet. Continuous inkjet printing dates back to at least 1929. See U.S. Pat. No. 1,941,001 to Hansell.
U.S. Pat. No. 3,373,437, which issued to Sweet et al. in 1967, discloses an array of continuous ink jet nozzles wherein ink drops to be printed are selectively charged and deflected toward the recording medium. This technique is known as binary deflection continuous ink jet, and is used by several manufacturers, including Elmjet and Scitex.
U.S. Pat. No. 3,416,153, which issued to Hertz et al. in 1966, discloses a method of achieving variable optical density of printed spots in continuous ink jet printing using the electrostatic dispersion of a charged drop stream to modulate the number of droplets that pass through a small aperture. This technique is used in ink jet printers manufactured by Iris.
U.S. Pat. No. 3,878,519, which issued to Eaton in 1974, discloses a method and apparatus for synchronizing droplet formation in a liquid stream using electrostatic deflection by a charging tunnel and deflection plates.
U.S. Pat. No. 4,346,387, which issued to Hertz in 1982 discloses a method and apparatus for controlling the electric charge on droplets formed by the breaking up of a pressurized liquid stream at a drop formation point located within the electric field having an electric potential gradient. Drop formation is effected at a point in the field corresponding to the desired predetermined charge to be placed on the droplets at the point of their formation. In addition to charging tunnels, deflection plates are used to actually deflect drops.
U.S. Pat. No. 6,079,821, which issued to Chwalek et al. in 2000, discloses a method and apparatus for a continuous ink jet printing system in which a continuous stream of ink is broken into droplets by the application of heat at a nozzle, and is deflected for the purpose of printing by an asymmetric application of heat at the same nozzle.
Drop-on-demand inkjet printers selectively eject droplets of ink toward a printing medium to create an image. Such printers typically include a printhead having an array of nozzles, each of which is supplied with ink. Each of the nozzles communicates with a chamber which can be pressurized in response to an electrical impulse to induce the generation of an ink droplet from the outlet of the nozzle. Many such printers use piezoelectric transducers to create the momentary pressure necessary to generate an ink droplet. Examples of such printers are present in U.S. Pat. Nos. 4,646,106 and 5,739,832.
While such piezoelectric transducers are capable of generating the momentary pressures necessary for useful drop-on-demand printing, they are relatively difficult and expensive to manufacture since the piezoelectric crystals (which are formed from a brittle, ceramic material) must be micro-machined and precision installed behind the very small ink chambers connected to each of the ink jet nozzles of the printer. Additionally, piezoelectric transducers require relatively high voltage, high power electrical pulses to effectively drive them in such printers.
To overcome these shortcomings, drop-on-demand printers that use thermally-actuated paddles were developed. Each paddle includes two dissimilar metals and a heating element connected thereto. When an electrical pulse is conducted to the heating element, the difference in the coefficient of expansion between the two dissimilar metals causes them to momentarily curl in much the same action as a bimetallic thermometer, only much quicker. A paddle is attached to the dissimilar metals to convert momentary curling action of these metals into a compressive wave which effectively ejects a droplet of ink out of the nozzle outlet.
Unfortunately, while such thermal paddle transducers overcome the major disadvantages associated with piezoelectric transducers in that they are easier to manufacture and require less electrical power, they do not have the longevity of piezoelectric transducers. Additionally, they do not produce as powerful and sharp a mechanical pulse in the ink, which leads to a lower droplet speed and less accuracy in striking the image medium in a desired location. Finally, thermally-actuated paddles work poorly with relatively viscous ink mediums due to their aforementioned lower power characteristics.
U.S. Pat. No. 5,880,759, which issued to Silverbrook in 1999, discloses a class of two-stage drop-on-demand printing systems in which a selection mechanism, which determines which nozzles on a printhead are to emit drops, and a separation mechanism, which ejects drops from the selected nozzles, are combined.
U.S. Pat. No. 6,276,782 B1 and U.S. Ser. No. 2001/0045973 A1 disclose a drop on demand ink jet printer wherein electrical pulses are provided to a thermally-actuated paddle and a heater that is adjacent a nozzle opening. The pulse to the paddle causes the paddle to immediately curl into position to cause local pressurization of the ink in a nozzle and a meniscus of ink develops at the nozzle exit opening. A heat pulse generated by an annular heating element adjacent the nozzle opening lowers the surface tension of the ink in the meniscus and also thus lowers the amount of energy necessary to generate and expel an ink droplet from the nozzle opening. The end result is that an ink droplet is expelled at a high velocity from the nozzle opening which in turn causes it to strike its intended position on a printing medium with greater accuracy. Additionally, the mechanical stress experienced by the thermally-actuated paddle during the ink droplet generation and expulsion operation is less than it otherwise would be if there were no heater for assisting in the generation of ink droplets. Consequently, the mechanical longevity of the thermally-actuated paddle is lengthened.
This invention uses a newly discovered type of microelectromechanical vibrating beam to break up an ink stream in a continuous inkjet printing system, or to eject drops in a drop-on-demand inkjet printing system. Such beams, which are composed of two or more layers of materials with different coefficients of thermal expansion, at least one of which is an electrical conductor, and which are attached to walls at both of their ends, have vibrational frequencies that depend in an unexpected and useful way on temperature. At relatively lower temperatures, the vibrational frequencies of such beams decrease as temperature increases. At relatively higher temperatures, the vibrational frequencies increase as temperature increases. Therefore, there is an intermediate temperature at which the vibrational frequency is a local minimum as a function of temperature, and thus is particularly stable against fluctuations in temperature.
By adjusting the beam""s temperature to be the temperature at which it is optimally stable to fluctuations in temperature, or by fabricating the beam in such a way that this temperature is the beam""s operating temperature, one can construct an oscillating member that will vibrate reliably at a given frequency with a relative stability in amplitude of motion, and which can aid in stream breakup or droplet ejection in an inkjet printing system or liquid moving systems such as a pump.
The objects, features and advantages of the present in invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.